1. Field of the Invention
The present invention relates to an optical position-detecting apparatus which is used in a machine tool or the like to measure a position.
2. Description of the Related Art
FIG. 1 schematically shows the construction of an example of a conventional optical position-detecting apparatus. The position-detecting apparatus has a detecting section 5 capable of being displaced relative to a main scale 2 mounted on a subject being measured (hereinafter referred to as "the subject of measurement"). The main scale 2 is formed with a transmission-type diffraction grating which extends throughout the measurement length of the main scale 2 (the measurement length lies in the longitudinal direction of the main scale 2). The diffraction grating is composed of a plurality of transmitting portions and a plurality of non-transmitting portions, which are repeatedly provided along the main scale 2. The detecting section 5 comprises a light source 1 for radiating a collimated rays onto the main scale 2, an index scale 3 formed with a transmission-type diffraction grating having the same pitch as the transmission-type diffraction grating of the main scale 2, and a photodetector 4 for performing the photo-electric conversion of light transmitted through the index scale 3 and for outputting a displacement signal.
The position-detecting apparatus having the above-described construction operates in the following manner. Collimated rays generated from the light source 1 are transmitted through the respective transmitting portions of the main scale 2 and the index scale 3. Then, the transmitted light rays become incident on the photodetector 4, and are subjected to photo-electric conversion. When the main scale 2 and the detecting section 5 have been displaced relative to each other in the longitudinal direction of the main scale 2, the relationship with which the transmitting and non-transmitting portions of the main scale 2 and those of the index scale 2 oppose each other changes periodically, thereby causing a periodical change in the quantity of light made incident of the photo-detector 4. In this way, a periodical displacement signal is obtained.
With the above-described conventional optical position-detecting apparatus, since the distance between the main scale 2 and the index scale 3 greatly influences the amplitude and the waveform of a displacement signal, it is necessary to provide a mechanism for always keeping constant the distance between the main scale and the index scale.